In telecommunications networks, codecs are devices that encode and decode voice signals transmitted over the network. Conventionally, uniform pulse code modulation (PCM) was used to encode voice sent over the telecommunications network. Uniform PCM involves sampling voice signals at a rate of 8,000 samples per second and 8 bits per sample, resulting in a 64 kbps codec rate. More recently, in mobile communications networks, adaptive modulation rate (AMR) codecs have been developed in which encoding and decoding rates change during a call. AMR is used to reduce the bandwidth used by voice calls.
One problem associated with using AMR codecs or other different types of codecs is that transcoding may be required when the source and destination devices use incompatible codecs. Transcoding is a process by which a voice signal encoded according to one rate and encoding standard is converted to another rate and another encoding standard. One problem with performing transcoding is that it can introduce latency and degradation in the voice signal being transmitted.
FIG. 1 is a block diagram illustrating transcoders performing transcoding of a speech signal in a telecommunications network. Referring to FIG. 1, a first transcoder 100 receives an AMR voice signal at an IuUP or NbUP interface of a 3GPP UMTS network. Transcoder 100 performs a transcoding operation by which the AMR voice signal is converted to PCM and forwards the signal to transcoder 102. Transcoder 100 introduces latency and voice degradation into the signal. The latency and voice degradation introduced by transcoder 100 is indicated by T1 in FIG. 1.
Transcoder 102 receives the PCM signal from transcoder 100 and performs a second transcoding operation, converting the PCM signal to AMR rate 1, the same AMR rate received by the first transcoder. Transcoder 102 introduces further latency and voice quality degradation into the signal. The latency and voice quality degradation introduced by transcoder 102 is indicated by T2 in FIG. 1. In the example illustrated in FIG. 1, because the ingress and egress AMR rates are equal, transcoding is unnecessary. However, transcoding is performed because no intelligence exists in the network illustrated in this example to eliminate transcoding.
In order to avoid the difficulties associated with transcoding, methods for transcoder-free operation have been developed. Transcoder-free operation refers to operation in which a connection that is established between telecommunications endpoints, such as mobile telephones, that have compatible codecs where the connection does not use transcoders. FIG. 2 is a block diagram of a conventional transcoder-free operation implementation developed by the assignee of the present application for use in a media gateway, referred to as the SanteraOne™ media gateway. Referring to FIG. 2, media gateway 200 includes a plurality of packet network interfaces 202 for interfacing with voice over IP devices, such as mobile phones 204, an ATM switching fabric 206, voice servers 208, a TDM matrix 210, and TDM network interfaces 212. ATM switching fabric 206 establishes connections between packet network interfaces 202 and voice servers 208. Voice servers 208 perform voice processing functions, such as transcoding, encoding, and decoding. In the illustrated example, each voice server 208 includes a DSP 214 that implements a codec function. TDM matrix 210 switches TDM channels between TDM network interfaces 212 and voice servers 208. TDM matrix 210 also includes an HDLC bus 216 that interconnects DSPs on different voice servers. TDM network interfaces 212 interface with TDM based telecommunications endpoints.
In the example illustrated in FIG. 2, in order to implement a transcoder-free connection, two codecs and two HDLC channels are used. That is, one DSP 214 on voice server 108 monitors the rate of an encoder used by a first telecommunications endpoint and the other DSP 214 on a separate voice server card monitors the encoding rate being used by the other endpoint. Rates and rate changes are communicated between the codecs using the HDLC connections. No transcoding is performed by either voice server because the ingress and egress codec rates are the same.
One problem associated with the transcoder-free operation of the media gateway 200 illustrated in FIG. 2 it requires separate DPSs to monitor each endpoint of the connection Another problem is that the DSPs must be interconnected using two HDLC connections. Establishing each HDLC connection requires complex connection establishment procedures.
Thus, in light of these difficulties associated with providing transcoder-free operation in media gateway, there exists a need for improve methods, systems, and computer program products for providing transcoder-free operation in a media gateway.